The secretion of succus entericus will be observed in conscious dogs provided with Thiry-Vella fistulae of proximal jejunum. Extracts of hog gut mucosa will be fractionated to determine whether their secretagogue activity for small gut secretion can be accounted for completely by their content of cholecystokinin, secretin or gastrin. Synthetic secretin, C terminal octapeptide of cholecystokinin and pentagastrin will be used as standard reference materials in appropriate assay procedures.